ABSTRACT: The extreme genetic variability of HIV presents a strong, albeit theoretical, challenge to the development of effective prophylactic vaccines. Most current research emphasizes development of appropriate formulations to reproducibly protect against infection or disease induced by homologous virus strains. Next, the question of whether this protection will extend to challenge strains from different viral sequence subtypes (or clades), in the same individual , must be critically assessed to assess the practical utility of these immunogens. The Molecular Biology Core will assist development and assessment of the breadth of protection afforded by experimental HIV vaccines. We propose to develop and characterize vaccine-based immunogens and simian-human immunodeficiency virus (SHIV) chimeras as challenge viruses suitable for assessment of the efficacy of env-based vaccines. We will generate challenge strains that encompass the breadth of antigenic diversity expected to be encountered in individual patients, epidemiologically linked infections, and epidemiologically unrelated strains from the same and distinct envelope sequence subtypes. In addition, we will evaluate viral sequences within animals infected in the course of vaccine experiments to determine the genetic features of breakthrough viruses, and the selective pressures imposed on these viruses that might differ from unvaccinated animals.